Polyaxial pedicle screw having a through bar clamp locking mechanism

ABSTRACT

A polyaxial orthopedic device for use with rod implant apparatus includes a screw having a curvate head, a cross bar mounting element having a socket into which the head of the screw is initially polyaxially nested. The cross bar mounting element further includes a vertical split which permits the socket to be expanded or compressed via the application of a corresponding force directed at the split. The cross bar mounting element further includes a pair of upwardly extending members which define a trough into which a cross bar element is positioned. The cross bar element includes features which permit the secure fixation of a rod thereto, as well as a selectively slideable element, such as a nut, which may be tightened to apply the compressive force necessary to compress the interior volume onto the head of the screw, thus locking the assembly in place.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a polyaxial pedicle screw and, moreparticularly, to a screw for insertion into spinal bone having apolyaxial coupling and locking mechanism for mounting a stabilizing rodto a sequence of vertebrae.

2. Description of the Prior Art

The bones and connective tissue of an adult human spinal column consistsof more than 20 discrete bones coupled sequentially to one another by atri-joint complex which consist of an anterior disc and the twoposterior facet joints, the anterior discs of adjacent bones beingcushioned by cartilage spacers referred to as intervertebral discs.These more than 20 bones are anatomically categorized as being membersof one of four classifications: cervical, thoracic, lumbar, or sacral.The cervical portion of the spine, which comprises the top of the spine,up to the base of the skull, includes the first 7 vertebrae. Theintermediate 12 bones are the thoracic vertebrae, and connect to thelower spine comprising the 5 lumbar vertebrae. The base of the spine isthe sacral bones (including the coccyx). The component bones of thecervical spine are generally smaller than those of the thoracic andlumbar spine. For the purposes of this disclosure, however, the wordspine shall refer only to the cervical region.

Referring now to FIGS. 1, 2, and 3, top, side, and posterior views of avertebral body, a pair of adjacent vertebral bodies, and a sequence ofvertebral bodies are shown, respectively. The spinal cord is housed inthe central canal 10, protected from the posterior side by a shell ofbone called the lamina 12. The lamina 12 includes a rearwardly anddownwardly extending portion called the spinous process 16, andlaterally extending structures which are referred to as the transverseprocesses 14. The anterior portion of the spine comprises a set ofgenerally cylindrically shaped bones which are stacked one on top of theother. These portions of the vertebrae are referred to as the vertebralbodies 20, and are each separated from the other by the intervertebraldiscs 22. The pedicles 24 comprise bone bridges which couple theanterior vertebral body 20 to the corresponding lamina 12.

The spinal column of bones is highly complex in that it includes overtwenty bones coupled to one another, housing and protecting criticalelements of the nervous system having innumerable peripheral nerves andcirculatory bodies in close proximity. In spite of these complexities,the spine is a highly flexible structure, capable of a high degree ofcurvature and twist in nearly every direction. Genetic or developmentalirregularities, trauma, chronic stress, tumors, and disease, however,can result in spinal pathologies which either limit this range ofmotion, or which threaten the critical elements of the nervous systemhoused within the spinal column. A variety of systems have beendisclosed in the art which achieve this immobilization by implantingartificial assemblies in or on the spinal column. These assemblies maybe classified as anterior, posterior, or lateral implants. As theclassifications suggest, lateral and anterior assemblies are coupled tothe anterior portion of the spine, which is the sequence of vertebralbodies. Posterior implants generally comprise pairs of rods, which arealigned along the axis which the bones are to be disposed, and which arethen attached to the spinal column by either hooks which couple to thelamina or attach to the transverse processes, or by screws which areinserted through the pedicles.

"Rod assemblies" generally comprise a plurality of such screws which areimplanted through the posterior lateral surfaces of the laminae, throughthe pedicles, and into their respective vertebral bodies. The screws areprovided with upper portions which comprise, or have mounted thereto,coupling elements for receiving and securing an elongate rodtherethrough. The rod extends along the axis of the spine, coupling tothe plurality of screws via their coupling elements. The rigidity of therod may be utilized to align the spine in conformance with a moredesired shape.

It has been identified, however, that a considerable difficulty isassociated with inserting screws along a misaligned curvature andsimultaneously exactly positioning the coupling elements such that therod receiving portions thereof are aligned so that the rod can be passedtherethrough without distorting the screws. Attempts at achieving properalignment with fixed screws is understood to require increased operatingtime, which is known to enhance many complications associated withsurgery. Often surgical efforts with such fixed axes devices cannot beachieved, thereby rendering such instrumentation attempts entirelyunsucessful.

The art contains a variety of attempts at providing instrumentationwhich permit a limited freedom with respect to angulation of the screwand the coupling element. These teachings, however, are generallycomplex, inadequately reliable, and lack long-term durability. Theseconsiderable drawbacks associated with prior art systems also includedifficulty properly positioned the rod and coupling elements, and thetedious manipulation of the many small parts in the operativeenvironment.

One such inferior solution is provided in the TSRH™ spine system,produced by Sofamor Danek™. This product, illustrated in FIGS. 4a and4b, comprises a fixed-axis screw 40 having a threaded shaft 42 and apair of upwardly extending members 46a,46b which define a U-shaped upperportion 45. The front and back faces 46a,46b (back faces not shown) ofthe upwardly extending members 46a,46b are splined with a radiallyextending ridges 48.

Referring now to FIG. 4b also, a fixed cross bar member 50, which ismounted in the trough of the U-shaped upper portion 45 and transverse tothe overall axis of the screw 40, is shown. The cross bar member 50includes a first end 52 which is threaded, onto which a nut 54 may beadvanced. The second end 56 of the cross bar member 50 comprises a hole58 through which a rod may be disposed. The diameter of the first end 52is less than the diameter of the second end 56. An annular member 60 isslideably mounted about the middle of the cross-bar member (at thejunction of the first and second ends) which has opposing faces 62 and64. The first face 62, which addresses the first end 52, has a splinedconformation, such that it may join with the splined faces 46a,46b ofthe U-shaped section of the screw 40 (the splining permits a rotationalvariety of interfacing angles). The second face 64 includes a groove 66in which the rod may nest when it is positioned through the hole 58.When the first end 42 of the cross bar member 50 is positioned in theU-shaped trough, and the nut 54 is advanced, the first face of the disc60 locks to the splined faces 46a, 46b of the screw 40, by virtue of themutual splines. The annulus 60 itself is then slideably pushed towardthe second end 56 of the cross-bar member 50 until the second face 64 ofthe annulus, and more particularly the groove 66, compresses the rod inthe hole 58 of the cross bar member 50. This tightening of the nut 54,therefore, locks the assembly together.

The TSRH™ spine system requires a multitude of cross bar lengths toaccount for different anatomies, which control the distance of the rodfrom the screw head. In addition, the fixed nature of the head of thescrew severely limits the freedom of the surgeon to angulate the screwsand to rotate the cross bar relative to the shaft portion. (The crossbar can only be rotated into a limited number of positions-all of whichare transverse to the head of the screw. In a fixed axis screw system,this means the cross bar is always transverse to the shaft.) Thislimitation requires that screws be left higher--more "proud"--than wouldotherwise be comfortable for the post operative patient.

It is, therefore, the principal object of the present invention toprovide a pedicle screw and coupling element assembly which provides apolyaxial freedom of implantation angulation with respect to rodreception.

In addition, it is an object of the present invention to provide such anassembly which comprises a reduced number of elements, and whichcorrespondingly provides for expeditious implantation.

Accordingly it is also an object of the present invention to provide anassembly which is reliable, durable, and provides long term fixationsupport.

Other objects of the present invention not explicitly stated will be setforth and will be more clearly understood in conjunction with thedescriptions of the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

The preceding objects of the invention are achieved by the presentinvention which is a pedicle screw and rod coupling mechanism having apolyaxially rotating screw head which is selectively lockable at adesired fixed position once a cross bar rod coupling mechanism issecured to it. More specifically, the present invention comprises, in afirst embodiment, a bone screw having a semi-spherical head. The shaftof the screw is threaded for insertion, and secure retention invertebral bone. The ballshaped head has a constant radius of curvatureover the curvate portion thereof, which forms a ball top end of thescrew. Onto the ball head of the screw is received a cross bar mountingelement, which is initially polyaxially disposed on the ball head.

In more detail, the cross-bar mounting element comprises a U-shapedmember having a pair of upright extending elements, definingtherebetween a trough, and a lower socket portion. With respect to thefront and rear faces of the extending members, however, a set ofradially extending ridges are disposed on the surface, therein providinga splined conformation. With respect to the socket portion, the crossbar mounting element receives the ball head of the screw into thesocket; the socket having substantially the same interior volume asnecessary to permit the ball to loosely rotate in the socket. The socketis further provided with a pair of opposing slots which render thesocket compressible (the deflection of the slots in a narrowingdirection causing a decrease in the total volume of the socket,permitting the crush locking of the ball in the socket. The means bywhich the slots are deflected and the socket volume reduced is describedmore fully hereinbelow). In a first embodiment, the pair of slots infact comprise a single transverse split in the cross bar mountingelement, beginning at the bottom (the lower entrance into the socket)and extending up beyond the socket, to a level beyond the bottom of thetrough defined by the upwardly extending members (the split laterallydividing the lower portions of the upwardly extending members). In asecond, alternative embodiment, the pair of slots extend down from thetop of the cross bar mounting element to a point below the maximumradius of the socket. In either embodiment, the application of acompressive force, applied against the opposing faces of the upwardlyextending members can, therefore, cause the slots to narrow, and thesocket to be compressed.

The cross bar member of the present invention may be substantiallysimilar to the cross bar element of the TSRH™, as described more fullyin the Background of the Invention. Alternatively, it shall beunderstood that any element which can securely retain a rod, and canapply a compressive force against the opposing faces of the cross barmounting element, is sufficient.

The assembly and implantation of the present invention is set forthhereinbelow. The surgeon prepares the pedicle site for receiving theshaft of the screw. Once the screw is implanted, the cross bar mountingelement is mounted to the ball head of the screw. At this point, thecross bar mounting element remains rotationally free to angulaterelative to the screw.

Given the cross bar element of the TSRH™, the next step in theimplantation process involves positioning the cross bar element alongthe rod, and placing the threaded first end in the trough of the crossbar mounting element. The polyaxial freedom of the cross bar mountingelement permits substantially greater ease of use as the surgeon placesthe cross bar element in the trough. Finally the surgeon tightens thecross bar element to the cross bar mounting element, thus applying acompressive force to opposing faces of the upwardly extending members.This compressive force causes the pair of slots to compressibly narrow,in turn causing the interior surface of the socket portion of the crossbar mounting element to collapse onto the exterior surface of the ballhead of the screw, thus causing the cross bar mounting element and thescrew to be locked in position (at the selected polyaxial angle).

Multiple screw assemblies are generally necessary to complete the fullarray of anchoring sites for the rod immobilization system, however, thescrew and coupling element assembly of the present invention is designedto be compatible with alternative rod systems so that, where necessary,the present invention may be employed to rectify the failures of othersystems, including the TSRH™, the implantation of which may have alreadybegun.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a top view of a human vertebra, which is representative of thetype for which the present invention is useful for coupling thereto arod apparatus;

FIG. 2 is a side view of a pair of adjacent vertebrae of the type shownin FIG. 1;

FIG. 3 is a posterior view of a sequence of vertebrae of the type shownin FIGS. 1 and 2;

FIGS. 4a and 4b are side views of the TSRH™ screw and rod couplingmechanism of the prior art;

FIG. 5 is a side view of a bone screw having a curvate head which is anaspect of the present invention;

FIG. 6 is a side view of the coupling element of the present invention,wherein critical interior features of the element are shown in phantom;

FIG. 7 is another side view of the coupling element of the presentinvention, taken from a point of view which is rotated 90 degrees fromthat of FIG. 6, and wherein interior features of the element are shownin phantom;

FIG. 8 is a third view of the coupling element of the present invention,taken from from the bottom of the element;

FIG. 9 is a side view of an alternative coupling element of the presentinvention, taken from the parallel perspective as FIG. 7;

FIG. 10 is a side perspective view of a fully assembled embodiment ofthe present invention comprising elements shown in FIGS. 4b, 5, and 6(and 7-8).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention will be described more fully hereinafterwith reference to the accompanying drawings, in which particularembodiments and methods of implantation are shown, it is to beunderstood at the outset that persons skilled in the art may modify theinvention herein described while achieving the functions and results ofthis invention. Accordingly, the descriptions which follow are to beunderstood as illustrative and exemplary of specific structures, aspectsand features within the broad scope of the present invention and not aslimiting of such broad scope.

Referring now to FIG. 5, a side view of the screw portion of the presentinvention, comprising a curvate head, is shown. The screw 120 comprisesa head portion 122, a neck 124, and a shaft 126. In FIG. 5, the shaft126 is shown as having a tapered shape with a high pitch thread 128. Itshall be understood that a variety of shaft designs are interchangeablewith the present design. The specific choice of shaft features, such asthread pitch, shaft diameter to thread diameter ratio, and overall shaftshape, should be made be the physician with respect to the conditions ofthe individual patient's bone, however, this invention is compatiblewith a wide variety of shaft designs.

The head portion 122 of the screw 120 comprises a semi-spherical shape,which has a recess 130 in it. It is understood that the semi-sphericalshape is a section of a sphere, in the embodiment shown the section isgreater in extent than a hemisphere, and it correspondingly exhibits anexternal contour which is equidistant from a center point of the head.In a preferred embodiment, the major cross-section of the semi-sphericalhead 122 (as shown in the two dimensional illustration of FIG. 5)includes at least 270 degrees of a circle.

The recess 130 defines a receiving locus for the application of a torquefor driving the screw 120 into the bone. The specific shape of therecess 122 may be chosen to cooperate with any suitable screw-drivingtool. For example, the recess 130 may comprise a slot for a screwdriver,a hexagonally shaped hole for receiving an allen wrench, or mostpreferably, a threading for a correspondingly threaded post. It isfurther preferable that the recess 130 be co-axial with the generalelongate axis of the screw 120, and most particularly with respect tothe shaft 126. Having the axes of the recess 130 and the shaft 126co-linear facilitates step of inserting the screw 120 into the bone.

The semi-spherical head portion 122 is connected to the shaft 126 at aneck portion 124. While it is preferable that the diameter of the shaft126 be less than the diameter of the semi-spherical head 122, it is alsopreferable that the neck 124 of the screw 120 be narrower than thewidest portion of the shaft 126. This preferable dimension permits thescrew to swing through a variety of angles while still being securelyheld in the socket of the cross bar mounting member (as set forth morefully with respect to FIGS. 6-9).

Referring now to FIGS. 6-8, a first embodiment of the cross bar mountingelement 200 of the present invention is provided in front side, lateralside, and bottom views, respectively. More specifically with respect tothe front side view shown in FIG. 6, the element 200 may be conceptuallyseparated into a lower socket portion 202 and an upper cross barreceiving portion 204. The lower socket portion 202 comprises acylindrical base having a semi-spherical interior volume 206 providedtherein. This interior volume 206 is accessible from the exteriorthrough a bottom opening 208. The ball head 122 of the screw 120 isinsertable into the socket 206, and is initially polyaxially rotatablewithin the socket through a wide range of angles which are limited onlyby the contact of the neck 124 of the screw against the lip 210 of theopening 208 (the diameter of the neck 124 necessarily being less thanthat of the opening 208).

The upper portion 204 of the cross bar mounting element 200 comprises apair of upwardly extending members 212a,212b which define therebetween aU-shaped trough 214. The front face of the upper portion 204 (the frontfaces 212a,216b of the upwardly extending members 212a,212b) includes aseries of radially extending ridges 218, which provide a splinedconformation thereon.

Referring now also to FIGS. 7 and 8, lateral side and bottom views ofthe cross bar mounting element 200 are provided. The lower socketportion 202, which includes the interior socket volume 206 necessarilyincludes a means by which the socket opening 208 may be expanded. Thismeans comprises a vertical split, or slot, 220 which extends upwardlyfrom the bottom of the element 200, into the upper portion. Morespecifically, the split 220 provides opposing slots on either side ofthe interior volume 206, and divides the upwardly extending members intofront and rear portions (215a and 215b, respectively, of upwardlyextending member 212b is shown in this lateral side view). The slot 220terminates at a point below the ultimate top of the upwardly extendingmembers 212a,212b, such that the cross bar mounting element 200comprises a single piece. The ball head 122 of the screw 120 may beinserted into the socket 206 when the split 220 is widened by theapplication of an outward tensile force directed on the front and/orback surfaces of the cross bar mounting element 200. Correspondingly,the application of an inwardly directed compressive force onto the frontand back surfaces of the element 200 causes the slot 200 to narrow, thuscausing the interior volume 206 to contract. This contraction causes thesurface of the interior volume 206 to crush lock to the surface of theball head 122.

Referring now to FIG. 9, an alternative embodiment of the cross barmounting element 250 is provided in a lateral side view. Thisalternative embodiment is substantially similar to the cross barmounting element 200, illustrated in FIGS. 6-8, differeing in thefollowing respects. First, the slot 254 extends from the top of theelement 252, down the upwardly extending members (shown here as dividingthe element into a front face portion 252a, and a rear face portion252b) to a position in the lower socket portion 258 which is below themaximum diameter of the socket 260. In order to load the screw 120, thisembodiment must also include an opening 256 in the top of the lowersocket portion 258. It shall be further understood that an outwardlydirected tensile force, applied to the front and rear faces of theelement, is required to permit the socket 260 to receive the the head122 of the screw 120. Further, the application of a compressive forceagainst the front and rear faces of the element 250 causes the socket260 to contract as before, which locks the head of the screw in thesocket. The cross bar member itself, which may comprise a variety ofembodiments, is contemplated to provide a rod receiving and securingportion, and a bar portion which seats in the trough 214 of the mountingelement 200 or 250 and provides a compressive force against the frontand rear surfaces thereof to lock the ball head 122 of the screw 120therein.

More particularly, with respect to the fully assembled embodimentillustrated in FIG. 10, in a perspective view, the cross bar and themethod of assembly of the entire system is described. First, the rodreceiving cross bar member 50, as originally shown in FIG. 4b, comprisesa first end 52 which is threaded (hidden in FIG. 10), onto which a nut54 may be advanced. The second end 56 of the cross bar member 50comprises a hole 58 through which a rod 57 may be disposed. The diameterof the first end 52 is less than the diameter of the second end 56. Anannular member 60 is slideably mounted about the middle of the cross-barmember 50 (at the junction of the first and second ends) which hasopposing faces 62 and 64. The first face 62, which addresses the firstend 52, has a splined conformation, such that it may join with thesplined front face of the upper section 204 of the cross bar mountingelement 200 (the splining permits a rotational variety of interfacingangles). The second face 64 includes a groove 66 in which the rod 57 maynest when it is positioned through the hole 58. When the first end 42 ofthe cross bar member 50 is positioned in the U-shaped trough 214, andthe nut 54 is advanced, the first face of the disc 60 locks to thespline 218 of the cross bar mounting element 200. Continued advancementof the nut 56 along the threading provides the compressive force (theannulus 60 and the nut 56 together) against the front and rear faces ofthe cross bar mounting element 200, thereby locking the element 200 tothe screw by virtue of the compressed slot and volume 206. Once thecompressive force of the annulus 60 and the nut 56 causes the slot 220to narrow and the head of the screw 120 to be locked in the socket 206,the annulus 60 itself is then slideably pushed toward the second end 56of the cross-bar member 50 until the second face 64 of the annulus, andmore particularly the groove 66, compresses the rod 57 in the hole 58 ofthe cross bar member 50. This tightening of the nut 54, therefore, locksthe assembly together.

While there has been described and illustrated embodiments of apolyaxial screw and coupling element assembly for use with posteriorspinal rod implantation apparatus, it will be apparent to those skilledin the art that variations and modifications are possible withoutdeviating from the broad spirit and principle of the present invention.The present invention shall, therefore, be limited solely by the scopeof the claims appended hereto.

We claim:
 1. A polyaxial screw assembly for use with orthopedic rodimplantation apparatus, comprising:a screw having a curvate head; across bar mounting element having upper and lower portions, said lowerportion including an interior socket, said interior socket being capableof polyaxially receiving therein said curvate head, and said upperportion including a pair of upwardly extending members definingtherebetween a trough, said cross bar mounting element further includingat least one vertical slot extending from said lower portion and intosaid pair of upwardly extending members of said upper portion, said atleast one slot rendering said interior socket contractible by theapplication of a compression force applied to said at least one slotwhich causes said at least one slot to narrow; and a cross bar assemblyhaving a first end, a second end, an elongate medial portion, and anannular member positioned about said elongate medial portion adjacent tosaid first end, said annular member having a diameter which is greaterthan a separation between the upwardly extending members of the crossbar mounting element, said elongate medial portion being positionable insaid trough of said cross bar mounting element such that said annularmember contacts a front face of said upper portion of said cross barmounting element, said first end including means for securely couplingto a rod; and a selectively advanceable means, which is selectivelyadvanceable along said cross bar assembly, from said second end towardsaid first end, said selectively means having a diameter which isgreater than the separation between the upwardly extending members ofthe cross bar mounting element and which selective advancement intocontact with a rear face of said cross bar mounting element applies acompressive force to said cross bar mounting element which in turncauses the at least one slot to narrow and the interior socket to becompressed and thereby compression lock to the curvate head of thescrew, securing the screw in the selected position relative to the crossbar mounting element.
 2. The assembly as set forth in claim 1, whereinthe at least one slot extends vertically from a position beneath a topof the upwardly extending members downward fully through the lowersocket portion.
 3. The assembly as set forth in claim 1, wherein the atleast one slot extends vertically from a position above a bottom of thelower socket portion upward fully through the upwardly extendingmembers.
 4. The assembly as set forth in claim 1, wherein said frontface of said cross bar mounting element on said upwardly extendingmembers includes a spline.
 5. The assembly as set forth in claim 4,wherein the portion of the annular member which contacts the front faceof said cross bar mounting element includes a corresponding spline.
 6. Aorthopedic rod implantation apparatus, comprising:at least one elongaterod; a plurality of polyaxial screw assemblies, wherein at least one ofsaid polyaxial screw assemblies includes a screw having a curvate head;a cross bar mounting element having upper and lower portions, said lowerportion including an interior socket, said interior socket being capableof polyaxially receiving therein said curvate head, and said upperportion including a pair of upwardly extending members definingtherebetween a trough, said cross bar mounting element further includingat least one vertical slot extending from said lower portion and intosaid upwardly extending members of said upper portion, said at least oneslot rendering said interior socket contractible by the application of acompression force applied to said at least one slot which causes said atleast one slot to narrow; and a cross bar assembly having a first end, asecond end, an elongate medial portion, and an annular member positionedabout said elongate medial portion adjacent to said first end, saidannular member having a diameter which is greater than a separationbetween the upwardly extending members of the cross bar mountingelement, said elongate medial portion being positionable in said troughof said cross bar mounting element such that said annular membercontacts a front face of said upper portion of said cross bar mountingelement, said first end including means for securely coupling to a rod;and a selectively advanceable means, which is selectively advanceablealong said cross bar assembly, from said second end toward said firstend, said selectively advanceable means having a diameter which isgreater than the separation between the upwardly extending members ofthe cross bar mounting element and which selective advancement intocontact with a rear face of said cross bar mounting element applies acompressive force to said cross bar mounting element which in turncauses the at least one slot to narrow and the interior socket to becompressed and thereby compression lock to the curvate head of thescrew, securing the screw in the selected position relative to the crossbar mounting element.
 7. The assembly as set forth in claim 6, whereinthe at least one slot extends vertically from a position beneath a topof the upwardly extending members downward fully through the lowersocket portion.
 8. The assembly as set forth in claim 6, wherein the atleast one slot extends vertically from a position above a bottom of thelower socket portion upward fully through the upwardly extendingmembers.
 9. The assembly as set forth in claim 6, wherein said frontface of said cross bar mounting element on said upwardly extendingmembers includes a spline.
 10. The assembly as set forth in claim 9,wherein the portion of the annular member which contacts the front faceof said cross bar mounting element includes a corresponding spline. 11.A polyaxial screw assembly for use with orthopedic rod implantationapparatus, comprising:a screw having a semi-spherical head; a cross barmounting element having upper and lower portions, said lower portionincluding an interior socket, said interior socket being capable ofpolyaxially receiving therein said semi-spherical head, and said upperportion including a pair of upwardly extending members definingtherebetween a trough and a spline on at least one set of exteriorfaces, said cross bar mounting element further including a pair ofopposing vertical slots extending from a point below a top of saidupwardly extending members downward fully through said lower portion,said pair of slots rendering said interior socket contractible by theapplication of a compression force applied to said slots which causessaid slots to narrow; and a cross bar assembly having a first end, asecond end, a threaded elongate medial portion, and an annular memberhaving a correspondingly splined lateral face directed toward saidsecond end, said annular member being positioned about said elongatemedial portion adjacent to said first end, said annular member having adiameter which is greater than a separation between the upwardlyextending members of the cross bar mounting element, said elongatemedial portion being positionable in said trough of said cross barmounting element such that said spline face of said annular membercontacts said splined face of said upper portion of said cross barmounting element, said first end including means for securely couplingto a rod; and a nut which is selectively advanceable along said threadedmedial portion of said cross bar assembly, said nut having a diameterwhich is greater than the separation between the upwardly extendingmembers of the cross bar mounting element and which selectiveadvancement into contact with a rear face of said cross bar mountingelement applies a compressive force to said cross bar mounting elementwhich in turn causes the at least one slot to narrow and the interiorsocket to be compressed and thereby compression lock to thesemi-spherical head of the screw, securing the screw in the selectedposition relative to the cross bar mounting element.